Alkaline electrolyte storage cells presently on the market are either of the "open" type (also referred to as being of the "industrial" type) enabling gas to be interchanged with the surrounding atmosphere, or else of the "sealed" type (also referred to as the "portable" type) having no interchange with the outside in normal operation. Most "sealed" type storage cells are intended mainly for incorporation in portable appliances; they are therefore small in size and of limited capacity. "Open" type storage cells are usually prismatic in shape, of high capacity, and low internal pressure. Storage cells of this type need to have their electrolyte level periodically readjusted because of losses by electrolysis and by evaporation, due to the contact with the ambient atmosphere. The increasingly tight requirements of users of "open" type storage cells have led to the development of storage cells that do not require maintenance, with this being done by reducing their interchanges with the outside.
The Ni-MH couple under development provides high performance, but it has been observed that it is necessary to put a strict limit on overcharging phenomena in such storage cells in order to guarantee satisfactory lifetime. For example, in the context of use for electric vehicle traction, the lifetime must be at least 1500 charge/discharge cycles. The performance of Ni-MH batteries depends on the charging method used enabling a maximum charge state to be achieved without accepting a large overcharge coefficient.
A maintenance-free alkaline electrolyte cell is charged in two stages. A first stage comprises charging proper, and corresponds to oxido-reduction of the active materials of the electrodes. For an Ni-MH storage cell, this reaction is slightly exothermal and it takes place without gas being given off. Once all of the active material of the positive electrode has been transformed, the cell enters a second stage known as the "overcharging" stage, during which oxygen is given off by the positive electrode. Electrochemical reduction of the oxygen at the negative electrode, or "recombination", leads firstly to an increase in temperature (exothermal reaction) which has the side effect of lowering the voltage of the cell, and secondly to increasing the internal pressure of the cell due mainly to the oxygen that is being recombined.
The higher the temperature at which the cell is being charged, the more progressive the changeover from the charging stage to the overcharging stage, thus making it that much more difficult to detect. Consequently, it is necessary to monitor permanently the parameters of a battery that is being charged.
Firstly, the charging method must make it possible to reach the maximum chargeable capacity with the best possible efficiency. Unfortunately, the chargeability of an Ni-MH cell decreases as its internal temperature increases. It is therefore necessary to define a method making it possible to optimize charging regardless of the initial charge state of the battery and regardless of the way its internal temperature varies during charging. The method may be included in the battery management system to enable the user to charge the battery without risk either for the environment or for the battery.
The charging method must also avoid allowing the internal pressure of the cell to increase excessively. In the event of excess pressure, safety valves open and as a result a loss of capacity is observed over cycling due to progressive drying out of the cell.
Finally, the charging method must minimize the duration of the overcharging stage. Overcharging is necessary to finish off the charging performed during the first stage, firstly to maximize the charge of the cell and secondly, in a battery of cells, to bring the various cells to a uniform charge level. The problem lies in selecting a reliable criterion for indicating the end of charging regardless of the initial conditions in which a battery was to be found. This criterion can only be based on the available physical parameters: voltage, pressure, and/or temperature.
The following end-of-charging criteria have already been proposed:
voltage drop (-.DELTA.V), which criterion is commonly used for the Ni--Cd couple with a switchover signal being generated conventionally at about -10 mV to -20 mV, this criterion is not suitable for application to the Ni-MH couple because of the small voltage signal generated by this couple at the end of charging (0 to -5 mV); PA1 the absolute increase in temperature (+.DELTA..theta.) of the cell between the beginning and the end of charging; this criterion is difficult to apply to the Ni-MH couple because temperature begins to increase as soon as charging begins; PA1 the relative increase in cell temperature compared with a reference heating relationship (.theta.-.theta..sub.ref), which method was adapted to the Ni-MH couple after the exothermal behavior of the charging stage of said couple had been modelled mathematically (FR-2 705 835); the switchover signal used is generally of the order of +10.degree. C. to +15.degree. C. for the Ni--Cd couple, but only of the order of +5.degree. C. to +6.degree. C. for the Ni-MH couple; and PA1 the rate of change of cell temperature, i.e. the time derivative of the temperature (+d.theta./dt), which criterion is frequently used for the Ni-MH couple because of the small size of the absolute temperature signal as generated by said couple at the end of charging; the switchover signal used is conventionally of the order of 20.degree. C. /hour to 60.degree. C. /hour. PA1 a "charging" first stage performed at a constant current I.sub.1 lying in the range I.sub.c /10 to I.sub.c /2 where I.sub.c is the current that would discharge said cell in one hour, during which stage the temperature .theta. of said cell increases; and PA1 a second "overcharging" stage performed at a constant current I.sub.2 lying in the range I.sub.c /50 to I.sub.c /10; changeover from said first stage to said second stage taking place when the time derivative of said temperature d.theta./dt reaches a threshold value (d.theta./dt), which varies as a function of the temperature .theta. of said cell at the moment of said changeover: (d.theta./dt).sub.s =f(.theta.).
All of those criteria relate to charging method adapted to cells of the sealed type and of small size, having a metal container that is generally cylindrical, and of small capacity (approximately up to 10 Ah). Those cells use a recharging method based on a high rate sequence (I.sub.c /2 to 2I.sub.c, i.e. charging at a rate that enables 100% of the capacity to be charged in 2 h to 1/2h). Since portable cells have low thermal inertia, they are very sensitive to variations in outside temperature. Their charging method can be interfered with by changes in the temperature of the environment in which they are placed (charging turned off too soon, charging not turned off at all, etc. . . . ). Documents DE-4 332 533, WO-92/11680, and WO-89/02182 have proposed taking account of any such possible fluctuations by using criteria based on cell temperature (+.DELTA..theta. and +d.theta./dt) together with a correction for ambient temperature.
Maintenance-free industrial Ni-MH cells have much larger capacities (10 Ah to 200 Ah) than do portable cells. Their rectangular shapes and the nature of their containers (plastics material) do not enable them to withstand significant excess pressure. Consequently, high rate charging governed by the above-mentioned criteria is not possible without running the risk firstly of the safety valves opening and secondly of significant heating, and that is prejudicial to the lifetime of the cell.